The present invention generally relates to systems for controlling contrast, and in particular to a system for controlling contrast in a liquid crystal display.
Liquid Crystal Displays (LCD) are widely used in consumer products such as computers, watches, refrigerators, calculators and video cassette recorders to name a few, they are also commonly used in man-machine interface (MMI) products as part of industrial control systems. LCDs are popular because they consume relatively little power, are inexpensive and allow a user to adjust brightness and contrast. Brightness control is necessary since the user may want to adjust brightness for factors such as ambient lighting conditions and user distance from the LCD. Contrast control permits the user to visually distinguish between foreground and background of the LCD.
Contrast of an LCD may be affected by operating temperature. As temperature decreases LCD fluid requires a higher voltage to maintain a desired contrast affect. Contrast voltage regulator circuitry also varies due to temperature and component value tolerances. Small changes in an output regulator, which drives the LCD, can visibly affect display contrast. In addition, there is a correlation between display brightness and contrast. Lower display brightness requires greater contrast in order to achieve the same level of viewability (e.g., a high-contrast, low-light output display can look similar to a low-contrast, high-light output display). Therefore, contrast control is desirable in that it provides compensation for these factors affecting display viewing properties.
LCDs employ a liquid crystal medium between two substrates. The liquid crystal medium may include various materials such as cholesterol, cyano-biphenyl groups, for example. When voltage is selectively applied to the two substrates, the arrangement of molecules of the liquid crystal medium is affected and hence contrast. The angle and direction in which the liquid crystal molecules orient themselves when the voltage is applied uniquely determines the magnitude of light transmittance. As the liquid crystal molecules are twisted with respect to an orthogonal direction (which defines maximum light transmittance), light transmittance varies. This phenomenon is marked in gray scale tones. The chemical makeup and composition of the liquid crystal medium is an element that determines the level of voltage applied. A voltage regulator circuit is typically employed (integrated within the display module) to establish the voltage applied to the LCD depending upon the contrast setting desired by the user. The voltage is typically controlled through a variable resistance provided by the user. The variable resistance is one element of a voltage divider used to bias the voltage regulator and hence, establish LCD contrast voltage. The resistance typically connects between a contrast node on the display (e.g., voltage regulator circuit within the display module) and common (or circuit ground).
Linear contrast control is generally desired because it allows the user to adjust contrast with ease and precision. For example, when a computer user adjusts a contrast control knob on a monitor, the user prefers a linear change between the foreground and the background as the knob is turned. Many conventional contrast control systems require complex and relatively expensive circuitry to afford for linear control.
In view of the above, a simple, relatively inexpensive system for providing a linear or other desired transfer function for contrast control is desired.
The present invention provides for a simple, relatively inexpensive, contrast control system. The system employs a plurality of parallel resistive elements, which may be selectively combined to provide a desired combined parallel resistance corresponding to a desired contrast setting. Switching individual resistive elements in or out of the combination affords for varying overall combined resistance. The employment of parallel resistive elements is highly conducive for a digital interface as compared to series resistive elements which often require employment of analog switching devices. The present invention employs N number of parallel resistive elementsxe2x80x94N being an integer greater than 1 and sufficient to effect a desired transfer function.
The input mechanism for contrast can be in substantially in any format that affects a unique value or condition that can be distinguished by the system. The means for affecting the value are numerous and range from simple manual switch inputs into the system, to computer control through communication channels or programmed algorithms, for example. A preferred format for the contrast value conducive to computer systems is a binary format. This format is defined with x bits where the total number of setting is 2x. For example, a 4-bit value will yield 16 settings. Another format example is through discrete input bits. Each input bit corresponds to a contrast setting, similar to a multiplexer. For example, 16 settings require 16 input bits. Of course only one bit can be active at a time as may be achieved by use of a selector switch.
One particular aspect of the system of the present invention includes a host processor, a decoder and a resistive system. The processor provides for operational control of the system. Once the processor determines a desired contrast setting, it sends to the decoder a binary value corresponding to the desired contrast setting. The outputs of the decoder are connected to the resistive system and provide for electrical connection of individual resistive elements to circuit ground. The resistive system includes a plurality of resistors integrated in parallel. The parallel connection of the resistors provides for easy coupling to the decoder, and no complex latch arrangements of conventional series type systems are required. The combined parallel resistance of the resistive system establishes a resistance corresponding to the desired contrast setting. The decoder is operative to switch in or switch out each of the resistors of the resistive system to effect the desired resistance value. The total resistance is used in conjunction with a display contrast driver thereby affecting a voltage to the LCD corresponding to the desired contrast setting.
One particular aspect of the, present invention provides for a parallel resistive system operative to facilitate controlling contrast of a display. A decoder is operative to receive a binary value corresponding to a desired contrast setting of the display. A plurality of resistive elements are configured to be selectively combined in parallel to provide a plurality of predetermined cumulative resistance values. The decoder selectively switches in and out the resistive elements to effect a cumulative resistance value which is employed to facilitate controlling contrast of the display.
Another aspect of the present invention provides for a system for controlling contrast of a liquid crystal display. A processor controls general operations of the system and is configured to determine a desired contrast setting of the LCD. The processor is operative to assign a binary value corresponding to the desired contrast setting. A binary decoder is operatively coupled to the processor and is configured to receive the binary value of x bits (x being an integer) from the processor. A circuit includes N number of resistors (N being an integer greater than the number of input bits) adapted to be coupled in parallel to effect a plurality of cumulative parallel resistance values for the circuit. The circuit is operatively coupled to the binary decoder. The binary decoder selectively combines the resistors to effect a specific cumulative parallel resistance value corresponding to the desired contrast setting.
Yet another aspect of the present invention relates to a system for controlling contrast of a liquid crystal display. The system includes means for determining a desired contrast setting of the LCD; means for assigning a binary value corresponding to the desired contrast setting; means for receiving the binary value; means for effecting a plurality of cumulative parallel resistance values; and means for selectively controlling the means for effecting a plurality of cumulative parallel resistance values to correspond to the binary value.
Still another aspect of the present invention relates to a system for controlling contrast of a display. The system includes means for determining a desired contrast setting for the display; and means for selectively combining at least a portion of a plurality of resistive elements in parallel to achieve a desired cumulative resistance value in order to facilitate effecting the desired contrast setting.
Another aspect of the present invention relates to a system for controlling contrast of a liquid crystal display (LCD). The system includes a processor configured to determine a desired contrast setting of the LCD. The system further includes a resistive system including N number of resistive elements (N being an integer greater than one), at least a portion of the resistive elements configured to be selectively coupled in parallel to effect a plurality of cumulative parallel resistance values, the resistive system being operatively coupled to the processor. The processor selectively combines the resistive elements to effect a specific cumulative resistance value corresponding to the desired contrast setting.
Another aspect of the present invention relates to a system for facilitating controlling contrast of a display, including N number of resistive elements (N being an integer greater than one), at least a portion of the resistive elements configured to be selectively coupled in parallel to effect a plurality of cumulative parallel resistance values. The system further includes a display driver operatively coupled to the resistive elements, the display driver driving the display to effect a contrast level corresponding to a specific cumulative resistance value obtained by a particular selective combination of the resistive elements, the selective combination ranging from one to N of the resistive elements.
Yet another aspect of the present invention relates to a method for controlling contrast of a display, comprising the steps of: determining a desired contrast setting of the display; combining at least a portion of resistive elements in parallel to effect a cumulative resistance value corresponding to the desired contrast setting; and driving a display to have a contrast setting corresponding to the cumulative resistance value.
To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.